Influence of Tropical Cyclones on the Marine Boundary Layer

Henry Potter & Tripp CollinsRaphael Ramos, Neil Williams, Will Drennan, Hans Graber

Abstract Materials & Methods Results Conclusions

Objectives Future Work

Acknowledgements

As part of the Impact of Typhoon on the Ocean in the Pacific (ITOP) a multi-institutional, multi-national field campaign, measurements from deep sea moorings in the South Pacific are used to investigate the effects of high wind speeds upon the marine boundary layer (MBL). Instruments amassed in situ data both above and below the water’s surface. Preliminary finding show typhoon generated wind speeds measured reaching ~30m/s, significant wave heights over 10m, a ~1˚C decrease in the upper ocean temperature, and 25mB drop in pressure.

Two tandem moorings of an Air-Sea Spar (ASIS) (Graber et al., 2000)* and an Extreme Air-Sea Interaction (EASI) buoy were deployed in a region of the South China Sea dubbed “Typhoon Alley”. Each pair continuously measured atmospheric and oceanic properties during the typhoon season in the Autumn of 2010.

ASIS and EASI with Research Vessel Roger Revelle after deployment in the South China Sea.

Both ASIS and EASI buoys were outfitted with instruments to measure wind speed and direction, wave height and direction, air and ocean temperature, humidity, aerosol size, radiation, ocean currents, air pressure, and mass fluxes. All buoys were furnished with sufficient batteries to power the instruments for ~100 days with recording rates up to 20 hertz. * Graber, H. C., Terray, E. A., Donelan, M. A., Drennan, W. M., Van Leer, J. C., & Peters, D. B. (2000). ASIS—A new Air–Sea interaction spar buoy: Design and performance at sea. Journal of Atmospheric and Oceanic Technology, 17(5), 708-720.

Our instruments recorded the passage of four tropical cyclones during the course of our campaign: Dianmu, Fanapi, super typhoon Megi, and Chaba. Although none of these directly hit our moorings, the effect of each can been seen in the instrument records. Initial results show increases in significant wave height (above 10m), wind speed (approaching 30m/s), drop in atmospheric pressure (over 25mB), and a decrease in mixed layer temperature (~1˚C) with tropical cyclone passage (see below).

The ITOP field campaign was an experimental achievement. Both moorings survived over 6 months afloat in the South China Sea and most instruments endured the relentless typhoon season. Results have shown we were able to successfully document characteristics of the MBL in some of the fastest wind speeds ever recorded at sea. Initial results have found significant wave heights above 10m, sustained wind speeds approaching 30m/s and a clear drop in atmospheric pressure and mixed layer temperature coinciding with tropical cyclone passage.

Special thanks for funding from the Office of Naval Research.

We seek to answer big questions relating to tropical cyclones:*Is deep ocean, high wind speed, boundary layer research viable?*How does the upper ocean respondto the passage of tropical cyclones?*What are the characteristics of wind and waves fields during tropical cyclones?*How do fluxes and aerosol size at the air-sea interface change during tropical cyclone conditions?

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Storm track of Typhoon Fanapi with estimated wind speed. Visible image of Typhoon Fanapi from 9/18/2010

Time series of wind speed, wave height, pressure, and upper ocean temperature for the North Mooring.

Dianmu Fanapi Megi Chaba

Further data processing will reveal greater details of the high wind speed MBL, including mass, momentum, and heat fluxes, current profiles, and aerosol sizes. Findings will be used to improve computer models of tropical cyclone intensity and tracking.